"It can thus be fairly concluded that the earth has an age of about 4600 M.y."

4.6 Gyr

Asimov, Isaac. The Measure of the Universe. New York: Harper Row, 1983: 256.

"… a variety of lines of evidence point to the Earth, and, indeed, the entire Solar System, as having been formed about 4.6 eons ago"

4.6 Gyr

The Earth is approximately 4.55 billion years old -- an inconceivable age when
one considers that the human being we would recognize as modern man has existed
for less than 50,000 years.

The oldest fossils of indisputable age found in Australia and South Africa
suggest that life (blue-green algae) existed between 3.46 and 3.47 billion years
ago. Radioactive dating methods give an age of up to 3.9 billion years for rocks
found on Earth. It only stands to reason that the Earth must be as old as anything
on it. It was from these numbers that estimates for the age of the Earth itself
were formed.

In 1956, Clair Cameron Patterson of the California Institute of Technology,
proposed the estimate for the Earth's age which today remains the accepted value.
He calculated the Earth as having been formed some 4.55 billion years ago (plus
or minus 0.07 billion years). Patterson worked under the assumption that the Earth
and the entire solar system were the same age. He argued that some meteorites
were formed 4.56 billion years ago and their debris constituted the Earth. The
Earth continued to grow through the bombardment of small, solid planetary bodies
until about 4.4 billion years ago when the Earth began to retain its atmosphere
and create its core.

Patterson used the uranium-lead clock and samples of lead isotopes found on
the Earth and in meteorites to come to this conclusion. The radioactive decay
of uranium 238 into lead 206 and uranium 235 into lead 207 is frequently used
to date rocks through the analysis of the daughter product, lead, of the parent,
uranium. Similar methods dated samples of rock from the moon at a similar age
of about 4.6 billion years.

The years that followed Patterson's work brought great change to the earth
sciences with the beginnings of studies of plate tectonics. Knowledge of the Earth's
past has changed and developed with time. Today it is less certain how the Earth
was formed than when it was formed.